Problem: If $9a + b + 8c = -6$ and $3x + 7y + 2z = -9$, what is $6z + 9x - 54a - 6b + 21y - 48c$ ?
Explanation: $= -54a - 6b - 48c + 9x + 21y + 6z$ $= (-6) \cdot (9a + b + 8c) + (3) \cdot (3x + 7y + 2z)$ $= (-6) \cdot (-6) + (3) \cdot (-9)$ $= 36 - 27$ $= 9$